Such an apparatus is known from U.S. Pat. No. 4,115,690. This apparatus has, inter alia, the following disadvantages:
1. The strip-shaped material is bent over repeatedly in the vicinity of the measuring instrument. The usually metallic material may be permanently deformed as a result. In this case the material is bent over repeatedly so that there is not only bending in one direction, but bending back and forth in both directions. This is also disadvantageous. If the material is then rewound onto a reel with torsion after measurement, detrimental forces can occur, as is known from twine or bobbins.
2. The measuring instrument rotates continuously. Therefore, the necessary inputs and outputs must be provided via slip rings.
3. The measuring time is limited. As beta radiation back scatter involves atomic statistics, the measuring precision can be increased by a sufficiently long measuring time. However, this is in principle impossible with the known instrument since the measuring time cannot commence until the material reaches the rotatable drum and must be terminated when the material is moved away from the measuring drum again. Therefore, in practice measurement is only possible over a time of about 270.degree. of drum rotation and absolutely impossible for about 90.degree. of the time.
4. Synchronizers must be provided to allow the measuring time to begin and end at the correct time.
5. In order that sufficient measured results may also be obtained, a plurality of beta emitters must rotate with the drum. However, to ensure that one beta emitter does not measure for a longer period than the others, the same length of measuring time determines that the beta emitters must be staggered through 180.degree. when there are two such devices, by 120.degree. when there are three beta emitters, etc.
6. In practice, at least two beta radiators must be provided to prevent the reading being merely a random test measurement.
7. The subsequently used computer must be equipped to process overlapping measuring times. In this respect also there are higher requirements than if only one beta radiator were to be used.
8. A really continuous reading is in this case merely replaced by a random test measurement, depending on the system. This method of measurement is not adapted to the measuring problem. If it is assumed, for example, that the instrument is intended for measuring copper-plated objects coming from a copper bath or gilded objects coming from a gilding bath, these units generally change their properties only at a slow rate, the time constants being in the region of at least a quarter of an hour. However, it is also characteristic for the coating to be suddenly defective at some point, but completely flawless to the left and right of said point. The known apparatus is not easily adapted for solving these problems.
9. If the beta radiator has detected a flaw, it must be determined which of the, if necessary, numerous beta radiators has detected the flaw.
10. The drum must have a comparatively large diameter to ensure careful treatment of the material. This creates design problems and, even when the instrument is installed in continuous production systems, this design results in disadvantages as there is usually very restricted space available in these systems.
11. If it is desired to measure strips or wires with the instrument, the known apparatus may be suitable as these materials are uniformly homogeneous. However, if it is desired to measure electric contacts which are in the form of punched-out strip, there is naturally sometimes one contact at one point and none at another point. It must therefore be ensured that the beta emitter is exactly opposite the contacts. This means that a quite special outer face of the drum is required for each measuring problem.
12. The known apparatus uses the known annular diaphragms or outer rings between the beta emitter and the material. Although these rings are made of a very hard material, they still wear. Moreover, particular care must be taken to ensure that the object to be measured is properly in contact with the annular diaphragms, otherwise erroneous readings are obtained.
13. Because the material is guided on the drum e.g. at 10 o'clock and guided off the drum at 7 o'clock, the first linearly advancing strip is not in alignment with the strip which is subsequently moved linearly off the drum. This staggered arrangement is in itself disadvantageous. However, as the material is under tension, this means that a component of a force is exerted on the apparatus at the same time.